How can I smooth the edges of these modelled letters?
For this type of illustration work you may want to consider using bezier curve objects instead, they allow fine grained control over the curvature of your objects. And you wont have to worry about triangulation or tessellation of the top and bottom faces.

They allow easier control of curvature and guarantee smoothness, and also allow easy non destructive beveling too.

Perhaps even draw them in a dedicated illustration program like Inkscape and import them as SVG into Blender

Inkscape Drawing


Imported into Blender


Be mindful about very tight or sharp corners inside the the meeting point of the K arms they may cause trouble with beveling

Also have in mind that they have downsides and will be more limited in other aspects if you later want to do more processing. They wont work well with subdivision surface, they have more limited texturing and material capabilities. Some of these limitations can be lessened by duplicating the objects and converting the copy to a mesh later.